1. Field of the Invention
This invention relates to a liquid filled suspension device which may properly change vibration damping characteristics and suspension characteristics, if necessary, as well as a control device for operating the liquid filled suspension device so as to selectively vary the vibration damping characteristics of the suspension device in accordance with the actual vibration conditions thereof. Such devices are applied to a suspension means for automobiles, a suspension means for automobile seats, a railway vehicle suspension means, an insulator means for machine tools, an insulator means for industrial machines and the like as well as an apparatus for the manufacture of integrated circuit (IC) and so on avoiding minute oscillations, and an insulator frame for precision apparatuses such as optical equipment and strain gauges and pumps.
As a matter of course, the liquid-filled suspension device may be applied alone or used together with a spring means such as coil spring, air spring or the like.
2. Related Art Statement
The conventionally known liquid-filled suspension device, for example, applied to automobiles is frequently provided with a shock absorber and a coil spring arranged therearound.
In this device, the shock absorber sealed with oil acts to damp vibrations, while the coil spring acts to support vehicle weight. In general, the vibration damping force is determined by a size of a connecting hole formed in a piston of the shock absorber, so that the size of the connecting hole is made small in case of requiring a large vibration damping force. Conversely, the size of the connecting hole is made relatively large when a large vibration damping force is not so required.
In order to sufficiently and effectively match the single suspension device against various vibration frequencies and amplitudes, however, it is required to properly change the vibration damping force of the shock absorber in accordance with the values of vibration frequency and amplitude. Lately, there are proposed some methods, wherein the size of the connecting hole formed in the piston is adjusted to a predetermined value by operating a slide plate driven by a motor or the like included in a piston rod, some of which have been put into practice.
However, in such conventional techniques, a mechanism for driving the slide plate is required to adjust the size f the connecting hole, so that there are many problems that the structure of the device becomes complicated, the size of the device is large, the weight of the device is heavy and the cost of the device is high.
In addition, the conventional device can not provide vibration damping characteristics followed to rapid change of vibration condition. Furthermore, even if the vibration damping force is properly controlled by changing the size of the connecting hole, there is naturally a limitation in the damping performance of the device. As a result, there is a problem that a peak having a vibration transmissibility of more than 1 is always produced in a resonance point of the vibration system and such a transmissibility can not sufficiently be reduced at a high frequency region.